The present invention generally relates to testing a semiconductor wafer and, more particularly, to measuring a total charge of an insulating layer of the semiconductor wafer using corona charge.
The production of insulating layers, particularly, thin oxide layers, is basic to the fabrication of integrated circuit devices on semiconductor wafers. A variety of insulating dielectric layers are used for a wide range of applications. These insulating layers can be used, for example, to separate gate layers from underlying silicon gate regions, as storage capacitors in DRAM circuits, for electrical device isolation and to electrically isolate multilayer metal layers.
The devices, however, are very sensitive to induced charges near the silicon surface. In most cases, device performance depends strongly on the concentration of free charges in the silicon. As a result, unwanted variations in device performance can be introduced by charges in the insulating layer and the insulating layer interface. The charges can result, for example, from static charging of the insulating layer surface, poorly forming the insulating layer, excessive ionic contamination within the insulating layer, and metallic contamination within the insulating layer. In addition to degradation of device performance, electrical isolation of individual devices can be impaired by unwanted surface channels due to induced charges. A property of increasing interest, therefore, is total charge Qtot or sometimes referred to as net charge Qnet of the insulating layer.
As illustrated in FIG. 1, there are five principle components of the total charge Qtot of an oxide layer: surface charge Qs; mobile charge Qm; oxide trapped charge Qot; fixed charge Qf; and interface trapped charge Qit. The surface charge Qs is charge on the top surface of the oxide layer and is frequently static charge or charged contaminants such as metallics. The mobile charge Qm is ionic contamination in the oxide layer such as potassium, lithium, or sodium trapped near the air/SiO2 interface or the Si/SiO2 interface. The oxide trapped charge Qot is electrons or holes trapped in the bulk oxide. The fixed charge Qf is charge at the Si/SiO2 interface. The interface trapped charge Qit varies as a function of bias condition.
Conventional methods of determining the total charge Qtot of an oxide layer include capacitance-voltage (CV), surface photovoltage (SPV) with biasing, and SPV analysis. The CV method typically measures each of the individual component charges, except the surface charge Qs which can be measured by the CV method, with a metal contact formed on the surface of the oxide layer and then obtains the total charge Qtot by summing up the individual component charges. The SPV with biasing method uses a contacting probe separated from the oxide layer with a Mylar insulator to bias the semiconductor. The total charge Qtot is determined by measuring the required bias of the probe to force a certain SPV. The SPV analysis method takes SPV measurements and infers the total charge Qtot via theoretical modeling.
While these methods may obtain the total charge Qtot, they each have drawbacks. The CV method requires expensive and time consuming sample preparation. The SPV with biasing method requires a contacting probe which can allow charge transfer from the oxide layer to the probe. The SPV analysis method relies on theoretical modeling and may not be extremely accurate. Additionally, the SPV methods only work over a narrow range of total charge Qtot, when the semiconductor is in depletion. Accordingly, there is a need in the art for an improved method of measuring the total charge of an insulating layer which is contactless, is a direct measurement with no theoretical modeling, is sensitive over a wide range of total charge, and is extremely accurate.
The present invention provides a method for measuring a total charge of an insulating layer on a substrate which overcomes at least some of the disadvantages of the above-noted related art. According to the present invention, the method includes depositing corona charges on the insulating layer and measuring a surface photovoltage for the insulating layer after depositing each of the corona charges. The method further includes determining a total corona charge required to obtain a surface photovoltage of a predetermined fixed value and using the total corona charge to determine the total charge.
According to one variation of the method according to the present invention, the total corona charge is determined by continuing to deposit the corona charges until the surface photovoltage measured is equal the fixed value. The total corona charge then corresponds to a sum of the corona charges deposited. According to another variation of the method according to the present invention, the total corona charge is determined using a data set of discrete points, preferably by interpolation. The discrete points include the surface photovoltages measured after each of the corona charges and corresponding total corona charges deposited to obtain each of the surface photovoltages.